Renowned science fiction author Douglas Adams died fifteen years ago and 25 May is the day fans choose to remember him – by carrying a towel.

If you’ve already encountered The Hitchhiker’s Guide To The Galaxy, you’ll know why. If you haven’t, don’t worry – you’ll find an explanation in the list below. (But you should probably read more.)

In the meantime, here’s a list, aimed at old and new fans alike…

1. Douglas Adams enjoyed a distinguished and varied career, but The Hitchhiker’s Guide To The Galaxy, and its various follow-ups, are his masterwork – telling the story of Arthur Dent, a man who explores the universe in his dressing gown, accompanied by a travel writer, a two-headed former Galactic President, a sparky astrophysicist and a terminally depressed robot.

2. Adams was renowned for his love of numbers – particularly 42, which he posited as the answer to the ultimate question of life, the universe and everything. (Despite years of fan theory, academic speculation and a number of false leads, we still don’t know the question.)

3. Lewis Carroll was also a big fan of the number 42 – it’s mentioned in The Hunting Of The Snark, and the combined age (in days) of the Red and White Queen may be reached by cubing 42. This may be a complete coincidence, but probability rules against it. (Oh, and there are 42 rules in cricket, a sport Adams frequently mentioned in his books.)

4. Besides the Hitchhiker’s Guide, Adams also created Dirk Gently, the world’s first (and last, and only) Holistic Detective. It’s an unusual concept, so by way of illustration, this is what really happened to Schrödinger’s Cat. (If you’ve never heard of Schrödinger’s Cat, consider yourself duly informed. But you should probably read more.)

5. Why a towel? Well, as Adams explains:

‘A towel…is about the most massively useful thing an interstellar hitchhiker can have. Partly it has great practical value. You can wrap it around you for warmth as you bound across the cold moons of Jaglan Beta; you can lie on it on the brilliant marble-sanded beaches of Santraginus V, inhaling the heady sea vapours; you can sleep under it beneath the stars which shine so redly on the desert world of Kakrafoon; use it to sail a miniraft down the slow heavy River Moth; wet it for use in hand-to-hand-combat; wrap it round your head to ward off noxious fumes or avoid the gaze of the Ravenous Bugblatter Beast of Traal (such a mind-bogglingly stupid animal, it assumes that if you can’t see it, it can’t see you — daft as a brush, but very very ravenous); you can wave your towel in emergencies as a distress signal, and of course dry yourself off with it if it still seems to be clean enough.’

6. They’re awkward things to carry around, though. So here’s a video demonstrating 42 ways to fold a towel, accompanied by soothing classical music and a collection of haiku. You’re welcome.

7. The 25 May has no special significance in Adams’ life; it was simply two weeks after his death, giving his fans time to spread the word, and it’s stuck. And Adams would almost certainly have appreciated the randomness.

8. Having said that, if you add the hexadecimal 25 and 5 – and then convert the result to decimal – the answer is 42. But of course, everyone knows that.

9. If you haven’t seen a sperm whale drop out of the sky and land on a mountain range, you haven’t lived. (And yes, that is Bill Bailey.)

10. Day off work? Why not visit Cambridge? Adams has strong links with the place – he was born there, attended university there, and set one of his Doctor Who stories there. (It’s lovely, particularly in summer.)

12. The Hitchhiker’s Guide To The Galaxy was a series of books, a TV programme, a video game and eventually a film – but before any of that, it was a radio play. The show still tours regularly, and has featured a number of special guests (including John Culshaw, Neil Gaiman and Phil Jupitus) providing the voice of The Guide.

13. One of Adams’ sci-fi creations – the Babel fish, a small aquatic mammal that fits in the ear and instantly translates everything into the wearer’s native language (thus breaking down language barriers and causing ‘more and bloodier wars than anything else in the history of creation’) has just become science fact with the invention of the Pilot, an earpiece translator. But before Google Translate got the monopoly on website translation, there was Yahoo! Babel Fish. It worked. Sort of.

14. Towel Day also shares its day with Geek Pride Day, so called because Star Wars was originally released on 25 May 1977. (But Towel Day is better. And it got there first.)

15. Why not celebrate with a poem or two? Vogon poetry is considered the third worst in the universe – but Earth has Julia A. Moore, the ‘Sweet Singer of Michigan’, whose turgid verse has delighted and appalled us for generations. Here’s one which deals (appropriately enough) with the subject of cricket.

16. Let’s deal with the sperm whale in the room: despite an impressive cast, the 2005 Hitchhiker’s Guide film is largely a freighter wreck. But Marvin – voiced by the late Alan Rickman – is by far the best thing in it.

17. One of Adams’ books was censored in the U.S. – Life, The Universe And Everything saw the word ‘f**k’ removed and replaced with ‘Belgium’, described as a small, flat country on Earth, but the most offensive word in the universe everywhere else.

18. Adams once played guitar at Earl’s Court with lifelong friend David Gilmour. If Pink Floyd rope you in as a special guest, you’ve probably made it.

19. As well as writing a string of novels and other stuff, Adams also found time (just about) to script-edit Doctor Who in the late 1970s, while Tom Baker was still in the TARDIS. While the stories produced under his reign were variable, his own episodes were top-notch – and he famously rewrote City Of Death in a single, coffee-and-whiskey-soaked weekend.

20. The Doctor Who references don’t stop with Tom Baker. Tenth Doctor David Tennant spent most of The Christmas Invasion (2005) in a dressing gown and pyjamas, giving Arthur Dent a mention at the end of the episode (and describing him as ‘a nice man’), having been revived by a cup of tea. In The Rings Of Akhaten (2013) the Eleventh Doctor shows Clara round an interstellar market and points out a Hooloovoo, and Fifth Doctor (and David Tennant’s father-in-law) Peter Davison appeared in the Hitchhiker’s Guide TV series, alongside his then-wife, playing a talking pig.

21. Towel Day is so popular worldwide that it’s even made it into orbit: here’s Samantha Cristoforetti, reading an extract from the first of the Hitchhiker books, while holding a towel.

22. You remember Doctor Snuggles, don’t you? You don’t? Oh well, never mind – just know that Adams co-wrote a couple of episodes. Voices by Peter Ustinov and John ‘Boycie from Only Fools and Horses’ Challis.

23. And if all else fails, there’s always a dolphin song and dance number.